http2: refactor request write flow

Move the entire request write into a new writeRequest function,
which runs as its own goroutine.

The writeRequest function handles all indefintely-blocking
operations (in particular, network writes), as well as all
post-request cleanup: Closing the request body, sending a
RST_STREAM when necessary, releasing the concurrency slot
held by the stream, etc.

Consolidates several goroutines used to wait for stream
slots, write the body, and close response bodies.

Ensures that RoundTrip does not block past request cancelation.

Change-Id: Iaf8bb3e17de89384b031ec4f324918b5720f5877
Reviewed-on: https://go-review.googlesource.com/c/net/+/353390
Trust: Damien Neil <dneil@google.com>
Trust: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

http2/client_conn_pool.go

@@ -84,6 +84,7 @@ func (p *clientConnPool) shouldTraceGetConn(cc *ClientConn) bool {
 }
 
 func (p *clientConnPool) getClientConn(req *http.Request, addr string, dialOnMiss bool) (*ClientConn, error) {
+	// TODO(dneil): Dial a new connection when t.DisableKeepAlives is set?
 	if isConnectionCloseRequest(req) && dialOnMiss {
 		// It gets its own connection.
 		traceGetConn(req, addr)

http2/pipe.go

@@ -30,6 +30,17 @@ type pipeBuffer interface {
 	io.Reader
 }
 
+// setBuffer initializes the pipe buffer.
+// It has no effect if the pipe is already closed.
+func (p *pipe) setBuffer(b pipeBuffer) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	if p.err != nil || p.breakErr != nil {
+		return
+	}
+	p.b = b
+}
+
 func (p *pipe) Len() int {
 	p.mu.Lock()
 	defer p.mu.Unlock()